1. Field of the Invention
The present invention relates to actuators. More particularly, the present invention relates to rotary actuators of the type which produce rotary motion of a shaft by means of a piston oscillating in a generally arcuate chamber.
2. Description of the Background
Rotary actuators are used in a variety of applications where it is desired to effect movement of a rotary fashion about a center point. For example, such actuators can be used to open and close valves, turn switches, operate steering mechanisms, etc. The actuator may be of the double-acting type wherein fluid, either hydraulic or pneumatic, is used to displace the piston in oscillating manner in an arcuate chamber to hence effect rotation of a shaft in a clockwise or counterclockwise direction depending upon movement of the attached piston. Alternatively, rotary actuators can be of the single-acting type wherein a fluid pressure is used to displace the cylinder and hence effect rotation of the shaft in one direction while rotation of the shaft in the other direction is accomplished by mechanical means such as a spring return automatically upon release of pressure acting on the piston.
A problem which has long plagued rotary actuators is sealing. Prior art rotary actuators are conventionally of a split body design in which the housing of the actuator is formed of two half sections, generally mirrors images of one another, which are adjoined such that a plane passing through the adjoined surfaces passes transversely, generally perpendicular, through the rotatable shaft journaled in the housing. In a double-acting actuator, it is necessary that the two halves of the housing be in fluid-tight sealing if the actuator is to operate efficiently. Further, in such double-acting actuators or for that matter even single-acting actuators, it is necessary that a seal be formed between the oscillating or reciprocating piston and the interior of the housing so that a fluid-tight chamber can be formed in which fluid pressure can be introduced to act on the piston and effect rotation of the shaft. The seal between the piston and the housing invariably lies in a plane which is generally perpendicular to the seal between the two mirror image halves of the housing. Accordingly, in such rotary actuator designs there are two inter-connecting seals, i.e. the seal between the piston and the housing and the seal between the two housing halves which further complicates an already complicated sealing problem.
In rotary actuators of the single-acting type, and as noted above, fluid pressure is used to effect movement of the piston in one direction while a mechanical system such as a return spring operates to automatically move the piston in the other direction when pressure is released thus effecting clockwise and counterclockwise movement of the shaft to which the piston is attached. Particularly when such single-acting actuators are used in connection with valves, such as butterfly valves, it is desirable that the torque characteristics of the actuator be known to facilitate matching of the actuator to the particular valve style. In particular, it is desirable to have a single-acting actuator in which the opening and closing torque exerted by the actuator and hence on the valve is substantially constant such that in matching the actuator to a particular valve, it will be known that throughout the full travel, i.e. from open to close, of the valve, the actuator has a constant torque rating.